This invention relates to microwave devices, and, more particularly, to a transition structure between a microstrip and a waveguide.
Microwaves are high-frequency electromagnetic signals that typically have frequencies in the 0.9-120 GHz (gigahertz) range. Microwaves may be propagated in several ways, including through free space and in or along confined carriers. Examples of confined carriers are solid metallic conductors and hollow waveguides. A microwave is propagated along the surface of a solid metallic conductor. A microwave is propagated through space but within a confined volume in a waveguide.
The selection of the best propagation path of the microwave involves a variety of considerations. However, in many microwave systems it is necessary to perform transitions of the propagation path. For example, antennas are used to receive or send microwave signals through free space and, thence, perform the transition to or from the confined carrier. In other cases such as within microwave amplifiers or other electronic signal processing equipment, the propagation of microwave signals must undergo transitions between solid conductors and waveguides.
Microwave transitions between solid conductors and waveguides (either solid conductor-to-waveguide or waveguide-to-solid conductor) have historically been accomplished with a physical interpenetration of the two. For example, a solid conductor may penetrate into the interior of a waveguide perpendicular to the direction of propagation of the microwave within the waveguide.
For many microwave systems, such as communications satellites, it is important to reduce the size and weight of microwave systems. Microwave systems with small solid conductors, termed microstrips or striplines, have been developed to produce microwave circuitry in planar configurations and to reduce the size of the microwave electronic circuitry to a size approaching that of microelectronic devices operating at conventional frequencies. The configuring of microstrip/waveguide transitions is more difficult in microwave circuitry of this type.
Microwave processing circuitry and microstrip/waveguide transitions have been integrated into xe2x80x9cmicromachinedxe2x80x9d devices such as those disclosed in U.S. Pat. No. 5,608,263. The micromachined architecture, while operable, offers opportunities for improvement. These existing microelectronic transition structures are difficult to handle and are not conducive to the production of large numbers of identical devices by batch processing. They require considerable care in the alignment of matching structures.
There is a need for an improved approach to the fabrication of a microstrip/waveguide transition structure that overcomes the drawbacks of the existing devices, and still permits the incorporation of circuitry for microwave signal processing. The present invention fulfills this need and provides additional related advantages.
The present approach provides a microwave microstrip/waveguide transition structure and a method for making such a structure. The transition structure permits active or passive microwave devices to be incorporated into the transition structure. The microwave device is substantially planar, except for the necessary thickness to accommodate the waveguide. The fabrication technique is fully compatible with microelectronic fabrication technology and permits the use of batch processing techniques. No alignment of separate subassemblies is required.
In accordance with the invention, a microwave microstrip/waveguide transition structure comprises a substrate, an elongated microstrip layer residing on a surface of the substrate, and an elongated integral hollow waveguide having a side, the waveguide residing on the surface of the substrate. The microstrip layer and the side of the hollow waveguide comprise a single continuous piece of metal, which may be elongated in a common direction.
One embodiment of the microwave microstrip/waveguide transition structure may also be described as comprising a single substrate, a microstrip layer residing on a surface of the single substrate, and an integral hollow waveguide residing on the surface of the single substrate. The microstrip layer and the hollow waveguide comprise a single continuous piece of metal and are each elongated in a common direction.
In any of these embodiments, an electronic device may be affixed to the substrate and/or disposed within the interior of the waveguide. The waveguide is normally rectangular in cross section, but may be of any operable shape. One side of the waveguide contacts the substrate, and is contiguous with the microstrip layer. The microstrip layer may be of any operable thickness and width, and the width typically increases from a small value remote from the waveguide to the width of the contiguous waveguide wall as the microstrip layer transitions into the waveguide wall.
The materials of construction may be selected from many different operable materials. The substrate may be, for example, a ceramic or a glass. The microstrip layer and waveguide may be made of metals such as titanium-tungsten plated with gold, chromium plated with gold, or chromium-copper plated with gold.
A method of making a microwave microstrip/waveguide transition structure comprises the steps of providing a substrate, depositing a metallic layer on the substrate, and depositing a metallic hollow housing continuous with a portion of a length of the metallic layer, thereby defining a metallic hollow waveguide bounded by the metallic layer and the metallic hollow housing and having a contained volume therewithin.
The waveguide is desirably formed integral with the microstrip by depositing a layer of metal over the substrate, and then a patterned layer of photoresist material overlying a portion of the length of metal. Additional metal deposited over the photoresist forms a three-dimensional metallic structure, overlying and enclosing the photoresist core. Openings are made through the metallic structure, to permit the photoresist to be removed thermally, chemically, or otherwise. The result is the hollow, precisely dimensioned waveguide continuous with the microstrip. The transition is accomplished along the length of the transition structure. If desired, microwave processing devices may also be deposited on the substrate, either inside or outside of the interior of the waveguide, in an appropriate sequence with the formation of the hollow waveguide.
The microwave microstrip/waveguide transition structure of the invention thus uses a single structure to accomplish the transition in a planar, lightweight configuration. It is not required to fabricate separate parts and then register and attach the parts together, which is often difficult when the parts are very small. Large numbers of the transition structures may be fabricated in batch-processing operations. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.